1. Field of the Invention
The present invention relates to an electrical heating cable, the power output of which is self-regulating as the result of the incorporation of a material with a positive temperature coefficient (PTC), as well as heating devices incorporating such cables.
2. Related Art
Parallel resistance semi-conductive, self-regulating heating cables are well known. Such cables normally comprise two conductors (known as buswires) extending longitudinally along the cable. Typically, the conductors are imbedded within a semi-conductive polymeric heating element, the element being extruded continuously along the length of the conductors. The cable thus has a parallel resistance form, with power being applied via the two conductors to the heating element connected in parallel across the two conductors. The heating element usually has a positive temperature coefficient. Thus as the temperature of the element increases, the resistance of the material electrically connected between the conductors increases, thereby reducing power output. Such heating cables, in which the power output varies according to temperature, are said to be self-regulating or self-limiting.
FIG. 1 illustrates a typical parallel resistance, semi-conductive, self-regulating heating cable 2. The cable consists of a semi-conductive polymeric matrix 8 extruded around the two parallel conductors 4, 6. The matrix serves as the heating element. A polymeric insulator jacket 10 is then extruded over the matrix 8. Typically, a conductive outer braid 12 (e.g. a tinned copper braid) is added for additional mechanical protection and/or use as an earth wire. Such a braid is typically covered by a thermo plastic overjacket 14 for additional mechanical and corrosive protection.
Such parallel resistance self-regulating heating cables possess a number of advantages over non self-regulating heating cables, and are thus relatively popular. For instance, self-regulating heating cables do not usually overheat or burn out due to their PTC characteristics. As the temperature at any particular point in the cable increases, the resistance of the heating element at that point increases, reducing the power output at that point, such that the heater is effectively switched off.
Further, due to this self-regulation of heating element temperature, it is often unnecessary to utilise “cold leads” with such heaters. Cold leads are often required in non-regulated heaters, as in a high temperature environment, the heating element may reach relatively high temperatures. Cold leads are connected to the ends of such non-regulated heaters to enable the heating element to be connected to the electrical supply without, for example, overheating the terminals or the supply. Cold leads typically take the form of relatively low resistance wires arranged to produce no appreciable heat. However, the fixing of the cold leads often involves costly labour. Further, the connection between the cold lead and the heater has a relatively high failure rate, due to the temperature gradient and thermal cycling experienced by the connection.
Consequently, as self-regulating heaters are typically arranged to operate within a safe temperature range, cold leads are not required.
However, parallel resistance semi-conductive self-regulating heaters do possess a number of undesirable characteristics.
The most common failure mode of parallel resistance self-regulating heaters is loss of, or reduction in, electrical contact between the power conductors and the extruded semi-conductive matrix forming the heating element. For example, differential expansion of the components and thermal cycling may lead to such failure or reduction in electrical contact. Such a reduction leads to electrical arcing within the cable, and a consequent loss in thermal output. The operational life of the product is thus dependant upon the bond between the conductors and the heating element.
Often the heating cable will be at a relatively low temperature (and hence low resistance) when initially energised. The low resistance will thus draw a high start up current when the cable is energised from cold. Consequently, circuit breakers intended to provide a first level of electrical safety (over current protection) must be sized to allow much higher currents (often by a factor of 6) than the normal run or operating current. This results in a lowering of circuit safety and over-sized switch gear and components.